1. Field of the Invention
The present invention relates to a lubricating oil composition which can be suitably used as hydraulic fluid, traction fluid, bearing oil, gear oil, engine oil, etc.
2. Description of the Related Art
Zinc dithiophosphates have been widely used in lubricating oils having requirements for the function of improving wear resistance and load carrying capacity, thermostability, and oxidation stability.
Recently, as the service environment of these lubricating oils has become severe, there has arisen a demand particularly for lubricating oils having such high thermostability and oxidation stability as to withstand use at high temperature.
As a means of improving thermostability and oxidation stability, in Japanese Patent Application Kokai Koho (Laid-open) No. 60-248796 proposed is adding zinc dithiophosphates into lubricating base oils together with other additives including nitrogen-containing copolymers, metallic detergent-dispersants, ashless detergent-dispersants, etc. However, such additives decrease load carrying capacity and wear resistance and cannot bring about sufficient increase in thermostability and oxidation stability, and therefore, the obtained lubricating oil cannot be prevented sufficiently from formation of sludge and coloration and discoloration.